Fan exhaust nozzle for turbofan engine

ABSTRACT

The cross sectional flow area of a fan discharge nozzle on one side of a central plane of an associated gas turbine engine power plant is greater than the corresponding flow area of the fan discharge nozzle on an opposite side of the central plane to compensate for the blockage of fan airflow by a pylon.

RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/513,707, filed on Aug. 31, 2006.

BACKGROUND OF THE INVENTION

This invention relates generally to aircraft gas turbine engines andparticularly to turbofan gas turbine engines.

The operation of turbofan gas turbine aircraft engines is well known.Such engines include a serial arrangement of a fan, a compressor, acombustor and a turbine (the compressor, combustor and turbinecomprising a “core engine”). Air admitted into the inlet of the engineis compressed by the engine's compressor. The compressed air is thenmixed with fuel in the engine's combustor and burned. The high-energyproducts of combustion of the burned airfuel mixture then enters theturbine with extracts energy from the mixture in order to drive thecompressor and fan. That energy extracted by the turbine above andbeyond that necessary to drive the compressor and fan, exits the engineat the core engine exhaust nozzle thereof, producing thrust which powersan associated aircraft. A much larger amount of thrust is produced bythe fan which takes in ambient air and accelerates and discharges suchair through a fan exhaust nozzle. The ratio of the volumetric flow ofair accelerated by the fan to that of the products of combustiondischarged from the core exhaust nozzle can be as high as 5-10:1 or evenhigher.

As aircraft gas turbine engines evolve, they have been required toproduce greater and greater quantities of thrust for powering largecommercial transport aircraft of ever-increasing capacity, as well as tooperate on as little fuel as possible to accommodate the ever-increasingrange requirements of such commercial transport aircraft. Recentdramatic escalation in the cost of jet fuel has made the requirements ofminimizing the fuel consumption of modern commercial gas turbineaircraft engines even more important.

For efficient operation of such aircraft gas turbine engines, that is,to minimize the amount of fuel required to generate a given amount ofthrust, it is necessary that the flow output of both the turbine and fanbe precisely controlled as to both speed and direction. Controlling thespeed of such flows is achieved in general by controlling the crosssectional flow areas of the core engine and fan exhaust nozzlesrespectively, by either optimally sizing fixed area nozzles for nominalengine operating conditions or employing variable area exhaust nozzleswhich can be adjusted in area for optimal flow throughout a range ofoperating conditions. The geometric shape of the exhaust nozzlesthemselves controls the direction of flow therethrough.

Both the fan and core engine exhaust nozzles are functionally defined bycomponents of the engine's nacelle. The nacelle includes a core cowlwhich provides an aerodynamically efficient cover for the core engineextending threrearound and terminating at the downstream end thereof atthe engine's exhaust nozzle. The nacelle also includes an outer fan cowlwhich surrounds the core cowl, enclosing the blades of the fan anddefining with the core cowl, an annular fan duct which terminates at thefan exhaust nozzle. Heretofore, the core cowl and fan cowl have beenconcentric to one another, that is, both such components have shared acommon longitudinal center axis such that the fan duct, from the faninlet to the fan exhaust nozzle is, for the most part, perfectlyannular.

The engine and nacelle are typically attached to the underside of thewing of commercial transport airplanes by a pylon which includes asupport beam extending generally from a structural member of theaircraft's wing through the nacelle fan cowl and core cowl to theengine's case. Typically this beam is covered by a fairing toaerodynamically smooth the flow around the beam. Thus, it will beappreciated that the pylon must necessarily extend through the fan ductbetween the fan cowl and core engine cowl. The fairing over the mountingbeam somewhat reduces the disturbance to the air flow through the fanduct caused by the pylon, and it has always been felt that thedeleterious effect of the pylon's presence in the fan duct was limitedto the resistance to the flow through the annular fan duct caused by thepylon.

DISCLOSURE OF THE INVENTION

The present invention is predicated upon the discovery that not onlydoes the obstruction posed by the pylon in the fan duct necessarilyrestrict fan duct flow thereby reducing the flow rate through the fanduct, but also causes a shift in the direction of the thrust associatedwith the flow through the fan duct, away from the pylon. That is,applicants have determined that the obstruction to flow through the fanduct posed by the pylon in that portion of the duct occupied by thepylon, causes a diametrically opposite portion of the fan duct toreceive greater flow therethrough. This imbalance in the fan flow,between the two opposed portions of the fan duct, results in a shift inthe direction of the net thrust produced by the fan from a directionparallel to the center longitudinal axis of the engine. Since optimal(minimal) fuel consumption of a gas turbine engine is generally achievedby maintaining the direction of thrust produced by the engine in adirection parallel to the longitudinal centerline of the engine, theshift in the vector direction of the engine's net thrust output mustnecessarily compromise (increase) fuel consumption.

To accommodate this imbalance in fan flow through the fan duct caused bythe obstruction offered by the pylon's presence in the fan duct, inaccordance with the present invention, that portion of the fan ductthrough which the pylon extends, on one side of a central plane of theengine, is made larger than that portion of the fan duct on the oppositeside of the central plane to make up for the restriction to fan air flowcaused by the pylon. In a preferred embodiment, the difference in areabetween the two fan duct cross sectional areas at a downstream portionthereof (i.e., at the fan exhaust nozzle) is equal to the crosssectional area of the pylon presented to the flow through the fan duct.This difference in area essentially eliminates the shift in thedirection of the net thrust produced by the engine for optimal (minimal)fuel consumption in the face of the restriction caused by the pylon.

The increase in fan exhaust nozzle area in that portion of the fan ductthrough which the pylon extends may be achieved in several ways. Forexample, the center longitudinal axis of the fan cowl may be offsettoward the pylon from the longitudinal center axis of the engine at adownstream portion of the cowl. Alternatively, the longitudinalcenterline of the engine's core cowl may be displaced away from thepylon, with respect to the engine's centerline, or, where clearancespermit, and where the pylon presents a large obstruction to the flowarea through the fan duct, the fan cowl may be shifted toward the pylonand the core cowl away therefrom.

It is estimated that the asymmetric distribution of the fan duct flowarea at the fan exhaust nozzle thereof will result in up to anapproximate improvement of up to 0.5% in total specific fuel consumptionwhich, when taken in the context of modem commercial aircraftconsumption of hundreds of thousands of gallons of fuel on an annualbasis, represents a significant improvement in the operational costsassociated with such engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial front elevation of a commercial transport aircraftpowered by a gas turbine engine of the type employing the fan exhaustnozzle of the present invention.

FIG. 2 is a sectional side elevation taken in the direction of line 2-2of FIG. 1.

FIG. 3 is an enlarged view of the power plant of FIG. 2 with portions ofthe nacelle thereof sectioned and broken away to show details of thepresent invention.

FIG. 4 is a rear elevation of the gas turbine engine power plant shownin FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a commercial gas turbine engine poweredaircraft includes a wing 10 having one or more gas turbine engine powerplants 15 mounted on the underside thereof by a pylon 20. As best seenin FIG. 3, gas turbine engine power plant 15 comprises a gas turbineengine 25 characterized by a longitudinal central axis 27 which lies ina horizontal (under normal operating conditions) central plane 29 of theengine. In a manner well known in the art, gas turbine engine 25includes a case 35 enclosing a compressor 40 (not shown), a combustor 45(not shown), and a turbine 50 (also not shown), the details of which arewell known in the art. As is also well known in the art, air enteringcompressor 40 through inlet 55 is compressed in the compressor, andenters the combustor where it is mixed with jet fuel and burned, theproducts of combustion (working fluid) flowing into turbine 50 whichextracts energy therefrom to drive the compressor and provide thrust forpowering the aircraft. The turbine also drives a fan 60 comprisingblades 65 of fixed or adjustable pitch. As blades 65 rotate, they takein ambient air, and accelerate the air to provide the majority of theuseful thrust produced by the engine. Typically, due to the much largerdiameter of the fan compared to that of the core engine, in modemturbofan engines, the volumetric flow through the fan can be as high as5-10 times the volumetric flow through the core engine or in some cases,even higher.

For purposes of maintaining a controlled flow of air, both around theoutside of the power plant and through the core engine, the engine andfan are surrounded by a nacelle 70 comprising a core engine cowl 75which surrounds the core engine and a fan cowl 80 disposed around theexterior of the core cowl and defining therewith, a generally annual fanduct 85 which accommodates the flow of ambient air accelerated by fan65, terminating at fan exhaust nozzle 86 at the downstream end of thefan duct. A tail cone 87 may be provided at the turbine exhaust nozzleto smooth the flow of working fluid exhausted from the turbine.

In a manner well known in the art, the engine 25 and nacelle 70 arefixed to the wing 10 of aircraft 5 by pylon 20. Pylon 20 is fixed to awing spar or other suitable structural component of the aircraft (notshown) at one end thereof, and, at the other end thereof, to the engineand nacelle. The pylon comprises a structural beam 90 which providessupport for the engine and nacelle, transmitting the weight andoperational (aerodynamic) loads thereof to the wing of the aircraft, anda faring 95 which provides a smooth aerodynamic contour to the pylon,reducing the aerodynamic losses associated with fan airflow therearoundas the power plant moves through the air with the aircraft.

It will be readily apparent that a significant amount of fan airflowthrough fan duct 85 will be blocked at an upper portion of the fan ductby pylon 95. It has long been recognized that the blockage of fan air inthe upper portion of the fan duct by the pylon contributes a significantamount of drag as the engine and pylon move through the ambient air.However, in accordance with the present invention, it has beendetermined that the flow restriction imposed on fan airflow through thefan duct in prior art gas turbine engine power plants also results in ahigher volumetric flow rate of air through the lower (away from thepylon) portion of the fan duct than the upper portion thereof. That is,the flow blockage in the fan duct associated with the pylon, causes anet migration of fan airflow from the upper portion of the fan duct, tothe lower portion thereof. As set forth hereinabove, for maximumefficiency, the thrust produced both by the fan and the core engineshould be directed parallel to the centerline of the engine. It has beendetermined that the greater flow through the lower portion of the fanduct skews the direction of the net thrust produced by the engine froman axial direction, thereby lowering the engine's efficiency so that thefuel consumption of the engine is increased from that which could be bya uniform fan airflow around the entire fan duct.

In accordance with the present invention, to compensate for the flowrestriction of the pylon in the upper portion of the fan duct, the crosssectional flow area of the fan exhaust nozzle at an upper portion(toward the pylon) thereof, i.e., that portion above the horizontalmid-plane of the engine is increased over the area of the fan exhaustnozzle below the engine (away from the pylon) mid-plane. This increasein the upper portion of the fan exhaust nozzle reduces the tendency ofthe fan flow to migrate toward the lower portion of the nozzle inresponse to the blockage associated with the pylon thereby correctingwhat otherwise would result in a vertical skewing of the net thrustproduced by the engine.

The area of the upper portion of the fan exhaust nozzle may be increasedin several different ways. For example, the downstream portion of thefan cowl may be vertically offset upwardly (toward the pylon) from aposition concentric with the core engine. That is, the downstreamportion of the longitudinal centerline of the fan cowl may be offsetvertically upwardly (toward the pylon) from the centerline of the coreengine at shown at 100 in FIG. 3. The increased flow area at the upperportion of the fan exhaust nozzle may also be achieved by offsettingcore cowl 75 downwardly (away from the pylon) such that the longitudinalaxis 110 thereof is offset downwardly from the longitudinal centerlineof the engine. Where a tail cone is used in the present invention, theextreme downstream-end of the core cowl should be made symmetric withthe core engine and tail cone so as not to vertically skew the directionof the thrust produced by the core engine.

The difference in cross-sectional areas between the two portions of thefan exhaust nozzle and thus, the amount of vertical displacement of thefan cowl and core cowl to achieve the increased area in the upperportion of the fan duct will, of course, depend upon the engine's thrustrating and by-pass ratio, dimensions of the pylon and dimensions andoperational parameters of the core engine and fan. In general, thelarger the engine, the larger the area of the pylon which partiallyblocks fan flow through the upper portion of the fan duct, therebyrequiring a larger increase in fan duct area over that which would berequired with smaller engines. While in the preferred embodiment, theincreased flow area in the upper portion of the fan duct is achieved byoffsetting the fan duct toward the pylon from the center line of thecore engine and offsetting the core cowl away from the pylon withrespect to the engine's centerline, it will be appreciated thatdepending upon the relative configuration of the engine nacelle andpylon, it may be possible to achieve the necessary increase in fanexhaust nozzle area by offsetting only one of these components from thecenterline of the engine.

While the fan duct and core cowl have been shown to be generallycircular in cross section as is normally the case for equalization ofaerodynamic loading therearound, it will be appreciated that for otherconsiderations, cross sectional shapes of these components may vary fromcircular. It will also be understood that although the pylon supportsthe engine from a location below the wing of the aircraft in theillustrative (preferred) embodiment, the present invention may beemployed with other configurations of power plants relative to theaircraft. For example, the invention herein may be used in aircraft inwhich the power plants are mounted above the aircraft's wings or on thesides of the fuselage.

Accordingly, while the invention herein as been described in referenceto a specific preferred embodiment, it will be understood that thosevariations thereof set forth herein as well as other variations andmodifications may suggest themselves to persons skilled in the art, andit is intended by the following claims to cover any such variations ormodifications as fall within the true spirit and scope of thisinvention.

1. A nacelle adapted for receiving therewithin, a gas turbine engine,said nacelle comprising: a fan duct; and a core engine cowl disposedradially inwardly of said fan duct, downstream portions of said fan ductand core engine cowl defining a fan nozzle therebetween; a first portionof said fan nozzle disposed on that side of a central plane of saidengine on which said pylon is disposed, having a larger flow area than asecond portion of said fan discharge nozzle disposed on the oppositeside of said central plane opposite that on which said pylon isdisposed.
 2. The nacelle of claim 1 wherein said central plane isoriented horizontally when at rest.
 3. The nacelle of claim 1 whereinsaid gas turbine engine has a central longitudinal axis associatedtherewith, a pylon which extends in part between said fan duct and coreengine cowl, said difference in cross sectional areas between said firstand second portions of said fan discharge nozzle being substantiallyequal to the component the area of that portion of said pylon extendingbetween said fan duct and core engine cowl, normal to the flow of airdischarged from said fan.
 4. The nacelle of claim 3 wherein said fanduct is characterized by a central longitudinal axis therethrough, saidfan duct longitudinal axis being at least in part offset toward saidpylon from said engine central plane.
 5. The nacelle of claim 3 whereinsaid core engine cowl is characterized by a central longitudinal axistherethrough, said core engine cowl longitudinal axis being at least inpart offset away from said pylon and said engine central plane.
 6. Thenacelle of claim 3 wherein each of said fan duct and core cowl havelongitudinal central axes therethrough, and at least one of said fanduct and core cowl longitudinal central axes at a downstream portionthereof being offset away from said engine central plane.
 7. The nacelleof claim 6 wherein said downstream portion of said fan duct longitudinalcentral axis is offset from said central plane of said engine in adirection toward said pylon.
 8. The nacelle of claim 6 wherein saiddownstream portion of said core cowl longitudinal central axis is offsetfrom said central plane of said engine in a direction away from saidpylon.
 9. The nacelle of claim 6 wherein said downstream portion of saidfan duct longitudinal central axis is offset in a direction toward saidpylon from said engine's central plane and said downstream portion ofsaid core cowl longitudinal central axis is offset from said engine'scentral plane in a direction away from said pylon.
 10. The nacelle ofclaim 1 wherein said fan duct at a downstream end thereof is generallycircular in cross section.
 11. The nacelle of claim 1 wherein said corecowl at a downstream end thereof is generally circular in cross section.